A block copolymer represented by the general formula A-B-A, wherein A means a styrene polymer block and B denotes a butadiene polymer block, an isoprene polymer block or a polymer block obtained by hydrogenating these polymers, is known to be a thermoplastic elastomer suitable for use as a pressure sensitive adhesive (see, for example, Handbook of Pressure Sensitive Adhesive Technology, the second edition, 1989, pp. 317-373).
Japanese Patent Publication No. 13594/1983 discloses a pressure sensitive adhesive composition of a pressure-sensitive hot-melt type, which comprises 100 parts by weight of a block copolymer represented by the general formula A-B-A, wherein A and B have individually the same meaning as defined above, 50-200 parts by weight of a tackifying resin (i.e., tackifer), 10-100 parts by weight of an extender oil, 5-35 parts by weight of low-molecular weight polyethylene and 1-20 parts by weight of low-density polyethylene or medium-density polyethylene. This pressure sensitive adhesive composition is described as being a pressure sensitive adhesive far excellent in heat resistance and good in electrical properties.
Japanese Patent Application Laid-Open No. 129085/1989 discloses a surface protective film making use of a pressure sensitive adhesive, which comprises a block copolymer represented by the general formula A-B-A, wherein A and B have individually the same meaning as defined above, a tackifying resin and polyethyleneimine in which a higher alkyl group has been introduced.
Pressure sensitive adhesive tapes or sheets comprise a base material and a pressure sensitive adhesive layer formed thereon. These tapes or sheets are generally wound up in the form of a coil with the pressure sensitive adhesive layer inside to form rolls. In these rolls, the pressure sensitive adhesive layer as an upper layer closely adheres to the back surface of the base layer as a lower layer under relatively strong adhesion. It is hence difficult to unwind or peel off the wound tape or sheet for use as things are.
Therefore, the back surface of the base material has usually been subjected to so-called releasability-imparting treatment so as to permit easy unwinding of the pressure sensitive adhesive tape or sheet without undergoing the migration to the back surface of the base layer as the lower layer and blocking of the pressure sensitive adhesive, and the delamination of the base material upon its unwinding. As such a releasability-imparting treatment, it has been often used to date to apply a release agent to the back surface of a base material in advance.
It is however very difficult to give to the release agent contradictory properties such that it is caused to firmly adhere to the back surface of the base material, but made liable to suitably separate from the pressure sensitive adhesive layer. Therefore, it is necessary to use a special release agent, to coat the back surface of the base material with a primer, or subject it to a corona discharge treatment prior to the coating of a release agent.
As the release agent, may be used linear alkyl-containing polymer (for example, polyvinyl ester allylcarbamates, silicone polymers, perfluoro hydrocarbon polymers, etc). In the method making use of the release agent, however, the cost of materials becomes more expensive by the cost of the release agent, and the production cost is also increased because of the required complex coating process. Moreover, even after producing the rolled pressure sensitive adhesive tape or sheet, there is a problem in quality aspect that the release agent migrates to the pressure sensitive adhesive layer, thereby causing the reduction of its adhesive strength.
The rolled pressure sensitive adhesive tapes making use of the pressure sensitive adhesive compositions described in Japanese Patent Publication No. 13594/1983 and Japanese Patent Application Laid-Open No. 129085/1989 also involve the same problems as described above. For example, when the pressure sensitive adhesive composition according to Japanese Patent Publication No. 13594/1983 is applied by hot-melt coating on kraft paper to form a roll of a kraft-based pressure sensitive adhesive tape, it is necessary to laminate polyethylene on the side opposite to the side of the kraft paper, on which the pressure sensitive adhesive is coated, i.e., the back surface, in advance and then apply a release agent thereto.